renice

What is renice?

renice is a command-line tool that allows you to change the niceness (priority) of running processes. Niceness is a numerical value that affects process scheduling. A lower niceness value means higher priority, while a higher value means lower priority. The default niceness is 0, but you can adjust it within a range of -20 (highest priority) to 19 (lowest priority). This doesn’t directly control how much CPU time a process gets, but it influences the scheduler’s decisions.

Understanding Niceness Values

The niceness value is an additive value. If you use renice to increase the niceness of a process, you’re adding to its current niceness value. For example:

• A process with a niceness of 0, having its niceness increased by 10 using renice, will now have a niceness of 10.
• A process with a niceness of 5, having its niceness decreased by 2 using renice, will now have a niceness of 3.

Using renice: Practical Examples

Let’s look at many practical scenarios and the corresponding renice commands:

1. Increasing the Niceness of a Specific Process (PID):

Let’s say you have a long-running process with PID 1234 that’s consuming excessive resources and impacting other applications. To lower its priority, you can use:

sudo renice 10 -p 1234

This command increases the niceness of process 1234 by 10. sudo is required because changing process priorities usually requires root privileges.

2. Increasing the Niceness of Processes Belonging to a User:

If you want to reduce the priority of all processes owned by a specific user (e.g., ‘john’), you can use the -u option:

sudo renice 5 -u john

This command increases the niceness of all processes owned by user ‘john’ by 5.

3. Decreasing the Niceness of a Process Group:

Sometimes, you might want to prioritize a group of processes related to a specific job. If you know the process group ID (PGID), you can use the -g option:

sudo renice -5 -g 5555

This command decreases the niceness of all processes in process group 5555 by 5, effectively boosting their priority.

4. Displaying Niceness Values:

To see the current niceness of a process, you can use the ps command with the -o option:

ps -o pid,ppid,ni,%cpu,%mem --sort=-%cpu

This command displays the process ID (PID), parent process ID (PPID), niceness (ni), CPU usage (%cpu), and memory usage (%mem), sorted by CPU usage in descending order. This allows you to easily identify resource-intensive processes and their niceness values.

5. Using renice with a shell script for automated tasks:

You can integrate renice into shell scripts to manage process priorities automatically. For example, a script might monitor CPU usage and automatically increase the niceness of processes exceeding a certain threshold.

#!/bin/bash


cpu_usage=$(ps -p 1234 -o %cpu | tail -n 1 | awk '{print $1}')


if (( $(echo "$cpu_usage > 80" | bc -l) )); then
  sudo renice 5 -p 1234
  echo "Increased niceness of process 1234"
fi

Remember to make this script executable (chmod +x your_script.sh) before running it.

These examples demonstrate the flexibility and power of the renice command for fine-tuning system resource allocation. Effective use of renice contributes to a more responsive and stable Linux system.